As dry development processes used in various electrostatic duplicating processes which have been put into practical use there have been known a two-component(binary) development process using a toner and a carrier such as iron powder and a one-component(unitary) development process using a magnetic toner comprising a magnetic material incorporated therein free of carrier The unitary development process using a magnetic toner requires no automatic concentration adjustor as required in the developing machine used in the binary development process. Thus, the developing machine used in the unitary development process can be compact. Further, since no carrier stain occurs, no maintenance such as replacement of carrier is required. Accordingly, the unitary development process has been used not only in low speed small-sized duplicating machines or printers but also in middle or higher speed duplicating machines, printers or plotters. Thus, further enhancement of properties have been desired in the unitary development process.
On the other hand, the binary development process employs a carrier which serves to agitate, carry and charge the developer separately of the developer and thus has a good controllability. Therefore, the binary development process has been widely employed. In particular, a developer comprising a resin-coated carrier is advantageous in that it has a good charge controllability and can attain relatively easy enhancement of environmental dependence and age stability.
In recent years, digitization has gone a long way in the field of printer as well as in the field of duplicating machine, making it possible to form a latent image more precisely, in particular, a minute difference in gradation with small kanji (Chinese character) or dots can be expressed. On the other hand, a plotter for a large-size drawing employing a magnetic unitary development process to produce a reduced sized plotter has been developed, A drawing is mainly composed of lines, and thus it is important to faithfully and stably reproduce the width of these lines. Digitization has made possible to form a latent image precisely. Thus, studies have been made on the faithful development of the high precision latent image.
As mentioned above, the magnetic unitary development process has various advantages. However, the magnetic unitary development process has essential problems from the standpoint of high image-quality development. In other words, the particulate toner undergoes magnetic agglomeration due to the magnetic material contained therein during development. Thus, the particulate toner seemingly increases in size, making it difficult to faithfully develop the latent image. This is a disadvantage which is not encountered in the binary development toner free of magnetic material.
The magnetic toner is also disadvantageous from the standpoint of fixability. In other words, the magnetic toner comprises a large amount of a magnetic material which cannot be fixed and thus is inevitably inferior to the nonmagnetic toner. Further, a magnetic toner which can be fixed with a lower energy has been desired.
On the other hand, a particulate toner comprising a polyolefin wax incorporated therein has frequently been used to eliminate various disadvantages in fixing properties such as offset in which the toner is attached to a heat roll used in the heat roll fixing process, causing stain on subsequent duplicating papers, smudge in which the fixed toner image is partially destroyed and transferred to a white paper when rubbed with the white paper and finger mark in which the fixed image is destroyed by a finger for peeling a paper which has been passed through a heat roll. The particulate toner comprising a polyolefin wax incorporated therein has a good releasability from the heat roller and hence a good offset resistance. However, since such a polyolefin wax has a poor compatibility with a binder resin, it forms a large domain in the binder resin. Thus, the toner can be easily destroyed at the domain portion during preparation, causing the wax to be exposed on the surface of the particulate toner. If such a toner is used in the magnetic unitary development process, the polyolefin wax migrates to the development sleeve and photoreceptor, causing the toner to be unevenly carried or the photoreceptor to be stained and hence causing density drop or image quality deterioration.
In the binary toner, too, a polyolefin wax is drastically exposed on the surface of the particulate toner. The polyolefin wax can migrate to the carrier or photoreceptor, causing density drop, toner scattering and image quality deterioration.
In order to eliminate such a secondary hindrance caused by polyolefin wax, an approach has been proposed which comprises specifying the amount of wax exposed on the surface of the toner as disclosed in JP-A-2-87159 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, this approach is disadvantageous in that the exposure of the domain of polyolefin wax cannot be thoroughly eliminated, making it impossible to eliminate the uneven toner distribution over the development sleeve. This rather worsens the offset resistance and deteriorates the fixability of the toner.
Attempts have heretofore been made to reduce the minimum dispersible diameter of polyolefin wax particles. For example, the use of a modified polyolefin wax has been proposed. JP-B-4-48227 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses the use of a modified polyolefin obtained by grafting a polyolefin with an unsaturated dicarboxylic ester. JP-B-4-30580 discloses the use of a modified polyethylene obtained by the block copolymerization of a polyethylene with an acrylate monomer made of acrylic ester or methacrylic ester. If only a modified polyolefin wax is used, the wax domain diameter is reduced. However, the resulting effect of raising the offset temperature is smaller than that of polyolefin wax. Thus, the added amount of the modified polyolefin wax needs to be increased. As a result, the amount of wax exposed on the surface of the particulate toner is increased, deteriorating the developability of the toner. Thus, no toners which can satisfy both the requirements for offset resistance and resistance to stain on the development sleeve have been found.
The combined use of a modified olefin wax and an olefin wax has been proposed in this respect as disclosed in JP-A-60-93456 and JP-A-60-93457. However, the effect of reducing the wax domain diameter is lessened depending on the mixing ratio of the modified olefin wax. This approach is also disadvantageous in that when the total amount of the two waxes based on the weight of the toner is increased, the amount of wax particles exposed on the surface of the particulate toner is increased, causing the wax to migrate to the development sleeve.